This invention relates to polyethylene foams made from blends of low density polyethylene and intermediate density linear polyethylene. It particularly pertains to improvement in processes whereby are obtained foams having substantially closed-cell structure and improved compressive strength.
The term "compressive strength" as used herein means the property of foam articles as determined by the test procedure described in ASTM D-3575-77, expressed in pounds per square inch (psi).
It is well known to make low density closed-cell ethylenic polymer resin foams by the process of extrusion foaming wherein a normally solid thermoplastic ethylenic polymer resin such as low density branched polyethylene is heat-plastified and mixed under pressure with a volatile material such as 1,2-dichlorotetrafluoroethane to form a flowable gel which is then passed through a shaping orifice or die opening into a zone of lower pressure. Upon the release of pressure, the volatile constituent of the gel vaporizes, forming a gas phase cellular structure in the gel which cools to a corresponding cellular foamed solid resin. Desirably, the resulting gas cells are substantially uniform in size, uniformly distributed through the foam body, and closed, i.e., separated from each other by membrane walls of resin. Although a number of general principles are thought to be understood, much of the extrusion foaming technology is empirical, based on experience, and directed to very specific materials and details to produce saleable products of narrowly defined specification.
However, attempts to make low density, i.e., less than 20 pounds per cubic foot, foam products from intermediate or high density linear polyethylenes have been met with numerous process difficulties.
It is also known to make polymer foams by the process of extrusion foaming utilizing blends of polyethylenes. In U.S. Pat. No. 3,496,124 polymer foams having high impact resistance are prepared from blends comprising from about 65 to about 80 parts by weight low density polyethylene, having densities within the range from about 0.89 to about 0.93 g/cc, and from about 35 to about 20 parts by weight high density polyethylene, having densities within the range from about 0.94 to about 0.97 g/cc. This reference does not contemplate the present invention: low density polyethylene comprises from about 40 to about 55 weight percent in the blend of the present invention, as compared to 65 to about 80 weight percent in the reference. Published Japanese patent application No. 47-56,497 filed June 8, 1972, disclose a polyethylene foam prepared from a blend of polyethylenes of two different densities. More specifically, polyethylene of density &lt;0.93 and of density &lt;0.94 are melt blended, pulverized, immersed in liquid blowing agents, and foamed at a temperature &lt;20.degree. higher than the arthmetical mean of the m.p. of the two polyethylenes. This reference does not suggest the use of intermediate density linear polyethylene in the polymer blend. U.S. Pat. No. 3,793,239 discloses a process for making foamed plastic films by extrusion foaming. More specifically, blends of polymers containing as a major component, a thermoplastic polymer of higher melting point than the crystalline polyolefin, the thermoplastic polymer of higher melting point being present from 1 to 15 percent by weight of the blend, and a two-component blowing agent system were used. Blends of high density polyethylene and polypropylene are preferred. This reference does not suggest the use of a low density polyethylene in the polymer blend.
U.S. Pat. No. 3,351,569 teaches a method for making fine-celled foam products by contacting crystalline polymer of ethylene or olefin in the pilot stage with a liquid blowing agent, provided that the polymers contain at least 10 percent by weight of a finely divided solid inorganic substance. The reference also discloses that mixtures of two or more of crystalline polymers of ethylene or propylene can also be used. The reference also discloses that a mixture of an aliphatic hydrocarbon and a fully halogenated alkane derivative containing both chlorine and fluorine can be used as blowing agent. However, this reference is limited to a method of making fine-cell foamed products, and, as such, does not contemplate the instant invention.
It is desirable that foam resin products have improved compressive strength at a given foam density, as compared to those products previously known.
It is, therefore, one object of this invention to provide improved foams of polymer resin blends. Another object is to provide a method and a means for making such foams. A particular object is to provide such improved method and a means for making polyethylene blend foams having substantially closed-cell structure and improved compressive strength per unit of density. Other objects and advantages of the invention are brought out in the description that follows.